Re: [libvirt] [Qemu-devel] [RFC] qmp: query-device-slots command

On 12/14/2016 12:17 PM, Marcel Apfelbaum wrote: > On 12/13/2016 06:00 PM, Eduardo Habkost wrote: >> On Tue, Dec 13, 2016 at 04:15:18PM +0200, Marcel Apfelbaum wrote: >>> On 12/13/2016 02:42 PM, Eduardo Habkost wrote: >>>> On Tue, Dec 13, 2016 at 12:04:17PM +0100, Markus Armbruster wrote: >>>>> Quick interface review only: >>>>> >>>>> Eduardo Habkost <ehabkost(a)redhat.com&gt; writes: >>>>> >>>>>> This adds a new command to QMP: query-device-slots. It will allow >>>>>> management software to query possible slots where devices can be >>>>>> plugged. >>>>>> >>>>>> This implementation of the command will return: >>>>>> >>>>>> * Multiple PCI slots per bus, in the case of PCI buses; >>>>>> * One slot per bus in the case of the other buses; >>>>> >>>>> Umm, that doesn't seem right. For instance, a SCSI bus has multiple >>>>> slots. The slot address is the SCSI ID. An IDE bus may have one >>>>> (SATA) >>>>> or two (PATA). For more examples, see qdev-device-use.txt section >>>>> "Specifying Bus and Address on Bus". >>>> >>>> Yes, I should have clarified that: this version changes only PCI >>>> to expose multiple slots, but other buses also need be changed to >>>> implement BusClass::enumerate_slots() properly, too. >>>> >>>>> >>>>>> * One slot for each entry from query-hotpluggable-cpus. >>>>>> In the example below, I am not sure if the PCIe ports are all >>>>>> supposed to report all slots, but I didn't find any existing >>>>>> field in PCIBus that would help me figure out the actual number >>>>>> of slots in a given PCI bus. >>>>>> >>>>>> Git tree >>>>>> -------- >>>>>> >>>>>> This patch needs the previous query-machines series I am working >>>>>> on. The full tree can be found on the git tree at: >>>>>> >>>>>> git://github.com/ehabkost/qemu-hacks.git >>>>>> work/query-machines-bus-info >>>>>> >>>>>> Example output >>>>>> -------------- >>>>>> >>>>>> The following output was returned by QEMU when running it as: >>>>>> >>>>>> $ qemu-system-x86_64 -machine q35 \ >>>>>> -readconfig docs/q35-chipset.cfg \ >>>>>> -smp 4,maxcpus=8,sockets=2,cores=2,threads=2 >>>>>> >>>>>> { >>>>>> "return": [ >>>>> >>>>> Are you sure >3000 lines of example output make sense here? >>>> >>>> I'm not. :) >>>> >>>> That's why I need feedback from the PCI experts. I believe most >>>> of the PCI ports on q35 accept only one device, I don't think of it as "one device" but as "only slot 0 can be used" (since, as Marcel pointed out, a different device can be placed on each of the 8 functions of that slot). > but I see no code >>>> implementing that restriction, and no obvious PCIBus or >>>> PCIBusClass field indicating that. >>> >>> Hi, >>> >>> Sadly there is no such restriction today as far as I know. >>> We trust the user that he knows what he is doing :) >>> and we let libvirt implement the restriction. >>> >>> I do agree we should fix that. >> >> OK, this matches what I saw on the current code. >> >> But I would like to understand what exactly should be the rules >> if we fix that. Should we report only 1 slot if >> pci_bus_has_pcie_upstream_port(bus)? >> > > We should report 1 slot for a bus exposed by a PCIe Root Port > or PCIe Downstream Port. > > Be aware that 1 slot doesn't necessarily refer to only one device, > we can have a multi-function device.

Is that a fixed thing? I mean, is it always possible to use all 8 functions of a slot (assuming no hotplug of course)?

> > Thanks, > Marcel > >> (Note that pci_bus_has_pcie_upstream_port() doesn't exist in the >> current code, it is added by my patch) >> >